The invention relates to a vehicle brake controller and a vehicle brake control method that adjust braking force of wheels provided in a vehicle.
Generally, in a case where lateral acceleration acting on a vehicle during turning is great, the center of gravity of the vehicle tends to move to the outer side of the turn, and vertical load of the wheels positioned on the inner side of the turn (hereinafter referred to as inner wheels) tends to be small. When the vertical load of the inner wheel becomes small, gripping force in the front-back direction and gripping force in the lateral direction of the inner wheels both decrease, and the vehicle easily may slip sideways. Further, even if a starting condition of an anti-lock braking control (hereinafter referred to as an ABS control) under such a situation is satisfied and slip ratio of the inner wheels is controlled, there is a risk of not being able to sufficiently stabilize the behavior of the vehicle due to a gripping force of the inner wheel that has been decreased by the reduction of the vertical load not being sufficiently recovered.
As a brake controller that can solve such a problem, a device described for example in Japanese Laid-Open Patent Publication No. 4-237660 has conventionally been proposed. In the brake controller, in a case where the starting condition of the ABS control is satisfied while the vehicle is turning, the stabilization of the behavior of the vehicle is attempted by differentiating a manner of control for the wheels positioned on the outer side of the turn (hereinafter referred to as an outer wheels) and a manner of control for the inner wheels. Specifically, in the ABS control for the outer wheels, brake fluid pressure in wheel cylinders corresponding to the outer wheels is controlled such that the slip ratio of the outer wheels becomes optimal. At this time, the brake fluid pressure in the wheel cylinder corresponding to the outer wheels (hereinafter referred to as an outer wheel fluid pressure) is detected.
With respect to this, in the ABS control for the inner wheels, a gain value that is set to a smaller value as a lateral acceleration becomes greater is calculated due to the vertical load of the inner wheel becoming smaller as the lateral acceleration becomes greater. A control fluid pressure for the inner wheels is calculated by multiplying the gain value to the detected outer wheel fluid pressure, and a brake fluid pressure in a wheel cylinder corresponding to the inner wheels is controlled to be the control fluid pressure for the inner wheels.
Incidentally, in the above solution (hereinafter referred to as a first solution), costs of the controller are high because pressure sensors for detecting the brake fluid pressure in the wheel cylinders are necessary. Further, the optimal slip ratio of a wheel is different depending on the μ value of the road surface that the wheel contacts. Due to this, for example, in a case where the vehicle runs on a road surface in which the μ value of the road surface that the outer wheels contact and the μ value of the road surface that the inner wheels contact are different, the slip ratio of the inner wheels does not necessarily become optimal even if the control fluid pressure for the inner wheels is set based on the outer wheel fluid pressure. This is because deceleration of a wheel changes depending on the μ value of the road surface that the wheel contacts, even if the brake fluid pressure in the wheel cylinder corresponding to the wheel is constant. When the slip ratio of the inner wheels is deviated far off from being optimal as above, the gripping force of the inner wheels also does not sufficiently recover.
Further, the vertical load of the inner wheels changes depending on the loading of the vehicle even if the lateral acceleration is constant. Due to this, the control fluid pressure for the inner wheels set without taking the loading of the vehicle into consideration is not necessarily optimal for the inner wheels at that time.
Thus, in recent years, a method for achieving stabilization of the behavior of a turning vehicle (hereinafter referred to as a second solution) in a method different from the above first solution has been proposed. In the second solution, it is determined that a vehicle is performing a high-G turning, in which the vehicle turns in a state under a great lateral acceleration when the lateral acceleration acting on the turning vehicle exceeds a high-lateral G determination value, and limit control to limit an increase in braking force of the inner wheel is started. Due to this, the decrease of the gripping force of the inner wheel is reduced, and the behavior of the vehicle is stabilized.
Even during the execution of the limit control, the ABS control is executed on the inner wheels if the starting condition of the ABS control is satisfied. In this case, the braking force of the inner wheels may be increased by the ABS control being executed.
In the above second solution, the decrease in the gripping force of the inner wheel is suppressed by maintaining the braking force of the inner wheels when it is determined that the vehicle is performing the high-G turning. In this case, if the driver starts a brake pedal operation after the vehicle has been determined to be performing the high-G turning, the braking force is not applied to the inner wheel. Due to this, although the stability of the behavior of the vehicle is ensured by executing the limit control, the actual deceleration of the vehicle becomes small compared to required deceleration corresponding to the amount of the brake pedal operation by the driver.